As well known, in gas turbines cooling air is extracted from the compressor. During steady state operation it is extracted for the purpose of cooling the turbine blading. During transient manoeuvres such as start-up and shut-down, typically a higher amount of air is extracted and released to the atmosphere to support stable operation of the gas turbine and to avoid, for example, stall in the compressor. This transient air extraction is called blow-off.
Generally, the flow of air is extracted through a bleed duct, or a slot, from the compressor annulus and gathered into a cavity. The cavity has a flange from which the flow is extracted into an external piping system of the engine. Such configuration is repeated at different circumferential positions. The flow is ducted via the piping system to the turbine for cooling or released to the atmosphere in transient manoeuvres.
The cavity is either formed by a casted space in the compressor casing or by an open space between the compressor vane carriers (often referred to as C.V.C.) and the compressor outer casing.
More in the specific, start-up conditions are potentially dangerous for a compressor because it operates far from design conditions. As a consequence, there is a large mismatch of flow, leading to large incidence angles on the blades and vanes. This results in flow separation. Moreover, at part-speed, the volume flow through the compressor gets too high due to an insufficient compression. This leads to an off-design axial velocity, which is not matched by proper rotational velocity of the blades.
In order to improve this phenomenon, the volume flow is reduced by extracting a large amount of air from the specified extraction points during start-up. It will then be purged in the blow-off system. Moreover, a fairly reduced amount of air is extracted from the main channel during normal operation for cooling purposes [1].
Shortly, the presence of an optimal compressor bleed cavity is vital to ensure that the above-mentioned processes during start-up and normal operations take place smoothly. An important aspect is the size of the cavity, which would have to contain the required amount of extracted air. The size of the cavity should compromise the extracted air both for start-up and normal operations.
However, constraints exist in terms of not only the functionality of the bleed system, but also of the entire structure and geometry of the overall compressor machinery, to achieve an optimal bleed flow.
For instance, a small bleed slot would be desired since it reduces the axial length of the compressor; and therefore reduces cost. Similarly, a small bleed cavity would reduce cost and compromise space constraint as well.
On the other hand, from an aerodynamic point of view, an optimal bleed system would be one that has low flow losses. This takes into account a larger view of the entire bleed system, from the air extraction point up to the piping lines.
The knowledge of flow pattern is essential to minimize losses due to the swirling and to enhance the geometry of the cavity, especially rounding at corners. As an example, FIG. 1 shows the flow topology in the bleed cavity, taking a rectangular form. The flow exits from the bleed slot into the bleed cavity, which immediately forms a swirl at the bleed exit due to the sudden turning of the flow. This forms an S-swirl with an adjacent swirl coming from the cavity bottom sidewall. The consequence of this S-swirl is a flow in the upward direction in the cavity, which then breaks into two 90° flows moving away from each other.
As the flows reach to the corner edges, they form a swirl and move vertically downwards again approaching the bottom corner edges, forming swirls. Then the whole flow topology repeats.
The configuration of the bleed cavity has an influence on the flow topology in the blow-off connection. The occurrence of different flow topologies in the bleed cavity is due to the flow going into the cavity from the bleed slot. In addition, the swirl system depends on the circumferential position, blow-off mass flow and the configuration of the cavity. The height of the cavity plays an important role in approximating the magnitude of friction losses along it. This could be obtained from a CFD calculation, which then further calculations on individual particle from different flow length could proceed, giving a better approximation of the friction loss.
Solutions known in the art tend to compromise between design and aerodynamic aspects as explained above to achieve the best possible design of the bleed and blow-off systems.
A critical point of the existing art is that, depending on the cavity width and flange position for the blow-off connection, a system of vortices is usually formed which results in high flow losses.